The invention relates to a zoom lens for an electronic camera with which still pictures can be picked up, which zoom lens, viewed from the object end, successively comprises a positive first lens group, a second negative lens group which is movable with respect to the first lens group, a third lens group and a stationary and positive fourth lens group, the fourth lens group being the main group which has a constant power and the first three lens groups combined constituting a lens system of small power and variable magnification.
A zoom lens of this type is known per se. U.S. Pat. No. 3,891,304 describes a zoom lens of the said composition for use in an 8-mm film camera. For a number of reasons the known zoom lens is not suitable for an electronic still picture camera.
In such a camera a photosensitive film is no longer used for recording the picked-up pictures, but these pictures are electronically recorded in order at a later stage to be reproduced via an electronic display device, such as a television picture display device, or printed. In this camera a so-called electronic image sensor is located in the image plane of the zoom lens, which sensor not only comprises a bidimensional matrix of radiation-sensitive semiconductor elements, but also electronic circuits coupled to these elements for processing and temporarily storing the picture information which is present in the electrical output signals of the radiation-sensitive elements. The image sensor has a radiation-sensitive surface whose dimensions differ from those of the image plane of a conventional photocamera or those of a conventional film camera. A commonly used electronic image sensor, for example in the form of a so-called charge-coupled device or CCD which is formed as a frame transfer device or FTD such as are used in video cameras has an image field diagonal of 11 mm. A zoom lens having a corresponding image field diagonal will have to be used when using such an image sensor in a camera for still pictures.
As compared with a zoom lens for a motion-picture camera, the lens for a still-picture camera should not only have a higher resolving power but it should also be free from distortion to a greater extent. Distortion is to be understood to mean the distortion of the image, for example the so-called barrel or pin-cushion distortion. The human eye can sooner detect a distortion or a poor resolving power in a still picture than in a moving picture.
As is known, an electronic colour image sensor comprises three types of radiation-sensitive elements each being only sensitive to the primary colours red, green and blue, respectively. These elements are juxtaposed in a regular pattern, with the elements of a first type always being surrounded by elements of the two other types. Since the radiation-sensitive elements have a given thickness, a beam portion having the size of a radiation-sensitive element may not only pass through a desired radiation-sensitive element but also through an adjacent element of another colour in the case of oblique incidence of the chief ray of an imaging beam. This might result in a colour shift in the reconstructed picture. It is therefore desirable for the zoom lens to be telecentric at the image end. In fact, the chief ray of a beam imaging a point of the object in a point of the sensor is then always perpendicularly incident on the sensor surface.